Special structures of microelectronic or micromechanical components often require that sublithographic structure elements be formed asymmetrically with respect to existing reference structures, for example on only one side of the reference structure, precisely aligned with the reference structure during the fabrication process.
The situation in which a sublithographic structure element of this type has to be arranged in the base region of a trench with a high aspect ratio (ratio of depth to width) is a particular problem. A structure element of this type can only be realized with very considerable restrictions using standard photolithographic patterning, but the other patterning methods which have hitherto been used for this purpose are reaching their limits in a number of respects when scaled to <90 nm.
Various solutions to the abovementioned problem are known, depending on the absolute feature size and the required accuracy of alignment between reference structure and structure element that is to be formed.
One possible option consists in accurate aligning of a lithographic mask relative to the reference structure and subsequent conventional patterning. The main problems of the method are the need to maintain the required accuracy of alignment between the underlying structure and the mask structure (with current exposure methods used in mass production, ≧30% of the minimum feature size) and the free exposure/developing of the regions which are not to be masked, in particular in the case of trench or relief structures with a high aspect ratio.
A further option is offered by what are known as shadow mask methods, which make use of a shadow effect of the reference relief.
In a first type, shadow mask coating of the relief is effected predominantly by vacuum vapor deposition of semiconductor or metallic materials from a directed areal, linear or punctiform source with the substrate in a stationary position. In this case, the mask layer is deposited substantially only in the unshadowed regions of the reference relief. The main problems of the method are the limited choice of materials, the availability of stable sources/installations and the need to maintain the required process conditions, such as for example a low residual gas pressure, a low substrate temperature, etc.
A second type provides for oblique implantation of ions, for example B or BF2, into a thin amorphous or fine-crystalline silicon layer that has been deposited on the reference relief, after which the unimplanted layer regions located in the shadow regions of the relief structure are removed by an isotropic selective silicon etching process. As a result, the mask layer remains in place in asymmetric form only in the irradiated regions of the reference relief. The thin silicon mask layer which remains is then optionally also converted into SiO2 by oxidation. The main problems of this type of process are the need to achieve the local implantation dose required for the selectivity of the silicon etch (approximately 1019 cm−3) in those regions of the three-dimensional structure of the reference relief which are to be masked, the restrictions imposed on the thickness of the amorphous silicon layer by the trench width of the reference relief, and the risk of parasitic doping of active silicon regions during the mask patterning.
A further type provides for oblique implantation of ions into a thin amorphous insulator layer deposited on the reference relief, and subsequent removal of the implanted layer regions by an isotropic selective etching process. As a result, the mask layer remains in asymmetric form only in the shadow regions of the reference relief.
The main problems of the method are the need to achieve the implantation dose required for the selectivity of the insulator etch (>1019 cm−3) in those regions of the three-dimensional structure of the reference relief which are to be removed, the relatively low selectivity which can be achieved with respect to implanted/unimplanted layer regions, the restrictions imposed on the thickness of the insulator layer by the trench width of the reference relief, and the risk of parasitic doping of active silicon areas during the mask patterning.
A fourth type provides for direct patterning with a thin, amorphous carbon mask layer deposited on the reference structure by oblique implantation of oxygen species (O, O2, O3).
As a result, the C mask layer remains in asymmetric form only in the shadow regions of the reference relief. This C mask layer structure is then transferred into further layers or into the substrate by means of conventional etching processes.
The main problems of this type are the availability of oxygen sources/installations with a sufficient jet intensity and a high productivity, making the C coating sufficiently conformal on the reference relief and the restrictions imposed on the C layer thickness by the trench width of the reference relief.